In industrial manufacturing and processing systems, especially those used in the manufacture of semiconductor components, it is often necessary to precisely measure vacuum or pressure at various points in the system by means of sensitive manometers or measuring instruments. The measurements produced by the instruments must be converted into electrical signals in order to remotely display the value of the measured variables or to provide inputs for automatic process control systems. In particular, in order to meet the needs of many modern processing systems the pressure or vacuum measuring devices must be able to accurately measure pressure or vacuum over a range of values spanning several orders of magnitude.
Several different types of prior art manometers have been developed to meet the range and electrical conversion requirements. Each of these prior art devices has specific limitations which cause it to be less than ideal in use in modern processing systems.
One well-known prior art analog gauge is the ion gauge which measures pressure or vacuum by monitoring an ionization current developed in a vacuum from a glowing filament. The ion gauge did not meet the range requirements stated above, for although it is suitable for high vacuums, it does not work well in low vacuum (less than 10.sup.-2 torr) environments.
A type of manometer which is suitable for both high vacuum, low vacuum and pressure environments is the capacitive manometer. This type of manometer usually consists of a transducer and a signal processing circuit to convert that resulting change in capacitance into an electrical signal. The transducer has a housing or casing that is divided into two sections by a thin metal diaphragm that is attached to the housing under tension. A port is provided in one section of the housing which can be connected to the source of pressure or vacuum to be measured. Variations in pressure or vacuum at the port are applied to the diaphragm to cause motion of the diaphragm. This motion varies the capacitance of a "capacitor" formed by the diaphgram and a fixed electrode attached to the housing wall.
Various methods have been developed to convert the change in capacitance to electrical which is suitable for use by the system which provides the process control. One such method has been to energize the variable capacitance produced by the pressure transducer with a frequency source that generates an electrical signal having a fixed frequency. Changes in the capacitance of the pressure transducer are thereby converted into changes in the current running through the capacitor. This current analog of the measured pressure can then be displayed on a meter or other display device or can be provided to processing circuitry.
Although the analog capacitive system works over the required pressure ranges, it has problems with linearity, thermal drift and line and "white" noise. Specifically, at low pressures (or high vacuums), the physical movement of the diaphragm is very small and thus the change in capacitance of the transducer is also very small. Since the signal is very small, noise and drift problems are exacerbated.
Prior art attempts have been made to eliminate some of the drift and noise problems by converting the analog signal into a digital signal and using this signal to drive display or processing circuitry. However, the analog-to-digital convertors have themselves produced noise and drift problems.
Other prior art attempts to remove noise and drift problems has resulted in a manometer which utilizes the transducer capacitance as a feedback element in a mechanical oscillator arrangement that includes the transducer diaphragm. The diaphragm is physically vibrated by means of external magnetic coils at a frequency which is dependent on the transducer capacitance. The output frequency is thereby also dependent of the value of the pressure applied to the diaphragm, since the apllied force changes the mechanical resonance of the diaphragm. This arrangement successfully eliminates some of the noise problem, however, requires a significant amount of energy to operate the driving coils located around the transducer diaphragm.
It is therefore an object of the present invention to eliminate noise and drift problems inherent in the prior art capacitance manometers.
It is another object of the invention to produce circuitry which directly presents the value of the transducer capacitance as a digital output.
It is a further object of the invention to utilize energizing circuitry which is simple enough to be mounted directly on the transducer unit itself so that the transducer unit provides a digital output signal which is less affected by noise and drift problems.